1. Field of the Invention
This invention concerns a device of emitting light by converting an electric energy into light and, more specifically, it relates to an organic light emitting diode which can be used suitably in the field of display devices, backlights, illumination light sources, exposure devices for use in electrophotography, marks and signboards.
2. Description of the Related Art
The light emitting diode is a self light emitting wholly solid device and since it has high visual recognizability and impact strength, a wide applicability is expected.
Those using inorganic fluorescent materials are predominant at present and used generally. However, since they require an AC voltage of 200 volts or higher for driving, they involve a problem of high running cost and insufficient brightness. On the other hand, study for charge light emitting diode using organic compounds has been started initially by using single crystals such as of anthracene. However, the film thickness is as large as about 1 mm and a driving voltage of 100 volts or higher is necessary. Therefore, it has been attempted to reduce the thickness by using a vapor deposition method (Thin Solid Films, 94, (1982) pp.171-183). The phenomenon of light emission of the devices described above is that electrons are injected from one of electrodes and holes are injected from the other of the electrodes to excite the light emitting materiel in the device to a high energy level and the excited luminescence material releases excess energy as a light when they return to the basic state. However, since the driving voltage is still as high as 30 V and the density of electrons hole carriers are low in the film and the probability of generating photons by carrier recombination is low, no sufficient brightness can be obtained and they have not yet been put to practical use.
By the way, Tang, et al. report an organic light emitting diode of a function separation type in which a hole transporting organic low molecular weight compound and a fluorescent organic low molecular weight compound having electron transporting function are laminated successively as extremely thin films by a vacuum vapor deposition method on a transparent substrate, which can provide high brightness of 1000 cd/m2 or more at a low voltage of about 10 V in 1997 (Appl. Phys. Lett., 51 (12), (1987) pp.913-915, and Japanese Published Unexamined Patent Application No. Sho 59-194393) and, from since, vigorous research and development have been conducted for organic charge luminescence devices. The light emitting diode of such lamination structure has a structure in which an organic light emitting material, a charge transporting organic material (charge transporting material) and electrodes are laminated in which holes and electrons move in the charge transporting material and emit light by recombination. As the organic luminescent material, for example, organic dyes emitting fluorescence such as 8-quinolinol aluminum complex or cumarin compounds are used. Further, the charge transporting material can include, for example, N,N-di(m-tollyl)N,Nxe2x80x2-diphenyl benzidine or diamino compounds such as 1,1-bis[N,N-di(p-tollyl)aminophenyl] cyclohexane and 4-[N,N-diphenyl)aminobenzaldehyde-N,N-diphenyl hydrozone compound.
However, although the organic light emitting diode has high luminescent characteristics, they involve a problem in the stability upon light emission or store stability. Since the layer formed with the organic material in the light emitting diode is as thin as from several tens to several hundreds namometers, the voltage applied per unit thickness is extremely high and the device is driven at a high current density of several mA/cm2, a great amount of Joule heat is generated. Therefore, it is often observed a phenomenon that the hole transporting low molecular weight compound or the fluorescent organic low molecular compound formed as films by vapor deposition in an amorphous glass state is gradually crystallized and finally melted to lower the brightness or result in dielectric breakdown and, as a result, the life time of the device is lowered. Further, it suffers from aging change and deterioration caused by oxygen-containing atmospheric gas or moisture during long time use.
In view of the above, it has been reported organic light emitting diode of using a star burst amine capable of providing a stable amorphous glass state as the hole transporting material (Extended Abstracts, 40th Annual Meeting of The Japan Society of Applied Physics, 30a-SZK-14 (1993)), or using a polymer formed by introducing a triphenyl amine to the side chain of polyphosphazene (Polymer Preprints, Japan Vol. 42, No. 7 (1993) pp. 2860-2862) for solving the problems in view of the heat stability of light emitting diode. However, since a energy barrier caused by ionizing potential of the hole transporting material is present, they cannot satisfy by themselves the hole chargeability form the anode or the hole chargeability to the light emitting layer. Further, in a case of the former star burst amine, purification is difficult due to the low solubility and it is difficult to enhance the impurity, and the latter polymer cannot provide high current density to obtain a sufficient brightness.
On the other hand, research and development have also been progressed also on the light emitting diode of a single layer structure intending to solve the problems described above and devices using conductive polymers such as poly(p-phenylene vinylene) (Nature Vol. 357 (1992) pp. 477-479) and formed by mixing an electron transporting material and a fluorescent dye in hole transporting polyvinyl carbazole (Extended Abstracts, 38th Annual Meeting of The Japan Society of Applied Physics 31p-G-12 (1991)) have been proposed but they are not still comparable with the laminate type light emitting diode using the organic low molecular weight compound in view of the brightness and the luminous efficacy.
Further, in the preparation method, a coating method is preferred in view of the simplification of manufacture, fabricability, increase of the surface area and it has been reported that devices can be obtained also by a casting method (Extended Abstracts, 50th Annual Meeting of The Japan Society of Applied Physics, 29p-ZP-5 (1989), and the 51st Annual Meeting of The Japan Society of Applied Physics, 28a-PB-7 (1990)). However, since the solubility and compatibility of the charge transporting material to the solvent and the resin are poor, they tend to be crystallized and have drawbacks in view of production or characteristics.
Further, Japanese Published Unexamined Patent Application No. Hei 11-135262 and No. Hei 11-246660 disclose organic electroluminescence material having a triaryl amine or heterocyclic amine structure.
This invention has been made in view of overcoming the foregoing problems in the related art, and this invention provides an organic light emitting diode having high luminous intensity, providing stable performance even after repeating use and easy to manufacture by using a charge transporting polymer excellent in stability upon light emission, store stability, solubility and compatibility.
The present inventors have made an earnest study on charge transporting polymers for solving the foregoing subject and, as a result, have accomplished this invention, based on the finding that a charge transporting polyether including repeating units containing, as a partial structure, at least one member selected from the structures shown by the following structural formulae (I-1) and (I-2) has a hole charging property, a charge mobility and thin film forming performance, which is suitable as an organic light emitting diode.
According to an aspect of the present invention, an organic light emitting diode has one or more organic compound layers put between a pair of electrodes including an anode and a cathode, one of which is transparent or semitransparent. At least one of the organic compound layers contains one or more kinds of charge transporting polyethers including a repeating unit containing, as a partial structure, at least one member selected from structures represented by the following general formulae (I-1) and (I-2): 
where Ar represents a substituted or non-substituted monovalent polynuclear aromatic ring having a number of aromatic rings of from 3 to 10, or a substituted or non-substituted monovalent condensed aromatic ring having a number of aromatic rings of from 2 to 10, X represents a substituted or non-substituted bivalent aromatic group, T represents a bivalent linear hydrocarbon group of 1 to 6 carbon atoms or a bivalent branched hydrocarbon group of 2 to 10 carbon atoms, m represents an integer of 1 to 3 and k represents 0 or 1.
An organic light emitting diode, according to another aspect of the present invention, has a hole transporting layer containing one or more charge transporting polyethers including repeating units containing, as a partial structure, at least one member selected from structures represented by the general formulae (I-1) and (I-2), and an emitting layer as the organic compound layer in this order on a transparent electrode.
The organic compound layer may be a single layer.
The charge transporting polyether may be a polyether represented by the following general formula (II): 
where A represents at least one member of the structures represented by the general formula (I-1) and (I-2), R represents a hydrogen atom, an alkyl group, a substituted or non-substituted aryl group or a substituted or non-substituted aralkyl group and p represents an integer from 5 to 5000.
The organic light emitting diode according to this invention has excellent stability during light emission, store stability and durability. Further, the organic light emitting diode according to this invention can provide an output at high brightness.